Executive Summary

Urine, which contains valuable nutrients like nitrogen and phosphorus, can be applied to soil and used as fertiliser for crops. Though valuable, urine is sometimes difficult to transport and store and it does not have a pleasant odour. To overcome these issues, struvite can be produced. Struvite (MgNH4PO4•6H2O) has many of the fertilising properties of urine with several advantages: the volume and weight are reduced, it can be stored in a compact form and it is easy to handle, transport and apply- especially in a granulated form.

How is Struvite Formed?

Urine contains phosphate (PO4) and ammonium (NH4). When magnesium (Mg) is added to urine, phosphate, ammonium and magnesium bind and form struvite (MgNH4PO4•6H2O), which can then be ﬁltered out, collected, and dried into a powder (see the figure above).

Where can Magnesium for the Precipitation be Found?

Various magnesium-containing substances can induce the precipitation of struvite- each with different qualities and costs.Bittern, the liquid, which remains after salt (NaCl) extraction from seawater, is highly concentrated with magnesium (and other minerals). Therefore, in coastal areas where salt is produced, this is an ideal source of magnesium. Furthermore, because it is a liquid, the bittern is easily dosed and readily mixed with urine. Magnesite rock (MgCO3) can also be used as precipitant if it is treated properly. The rock powder is heated in a calcination kiln (analogous to lime calcination) to produce magnesium oxide (MgO), which dissolves in urine (untreated MgCO3 will not dissolve). If no local magnesium source is available, other magnesium compounds (e.g. magnesium chloride, MgCl2.6H2O or magnesium sulphate, MgSO4•7H2O), may be purchased, though they are likely to be more expensive.

How is the Struvite Reactor Built?

A struvite reactor can be easily built, using locally available materials and skills (see the figure below). The main tank can be constructed with conventional plastic containers (e.g. polyethylene, polypropylene, polyvinylchloride) or galvanised steel sheets. The stirring mechanism, as well as the supporting structure should be assembled from steel, which is coated or galvanised for rust proofing. Pipes and fittings should be made of plastic to prevent corrosion.

How does the Struvite Reactor Work?

The reactor consists of a stirring mechanism, which is fitted inside the tank; a nylon cloth filter bag hangs below a valve to allow the main reactor to be drained (see the figure below). To start the process, the collected urine and magnesium are mixed for 10 minutes in the reaction tank. The valve is opened and the suspension is then drained into the filter bag. The filter bag retains the struvite while the effluent passes through. The filter bag is air dried for one to two days, after which point the struvite is ready to use. In field experiments, this type of reactor was able to recover over 90 % of the phosphate contained in the urine. Because struvite also precipitates naturally from urine, any precipitate in the collection system should be incorporated into the final product, in order to maximise nutrient recovery.

How can Struvite be Applied?

Struvite can be applied to fields just like any other fertiliser. In a simple granulation drum (see the figure below), the powder can be transformed into granules. In granular form, a fertiliser is easier to apply and does not cake in moist environments. Struvite performs comparably to diammonium phosphate (DAP) fertiliser and has the following advantages:

Applicability

Struvite precipitation may be appropriate for any situation where significant quantities of urine can be collected. Struvite can be produced from a variety of wastewaters (including domestic wastewater or liquid animal manure) but the process is more difficult and requires additional chemicals for pH control. Urine collection (labour) and transport (fuel) accounts for a large proportion of the costs, and therefore struvite production is more appropriate for areas where large volumes of urine are available within a small area; public toilets, schools and stadia are promising sources of urine. Magnesium should be available in a suitably soluble form, in sufficient quantities and at an affordable price to allow for an economic operation of the struvite production plant. Whereas direct application of urine may face low social acceptance, struvite, as a urine-derived but odourless product is generally well accepted.

The aim of this publication is to present and discuss the design drawings, from which a struvite precipitation reactor can be built. In addition, a breakdown of the building costs and some suggestions for improvements are presented.